1. Field of the Invention
This invention relates to a method of high-temperature machining, and more specifically to a method of forming apertures or windows in materials with a coherent beam of electromagnetic energy. Even more specifically, this invention relates to the formation of tapered apertures in thin dielectric or metal films with the coherent light of a laser.
2. Description of the Prior Art
In many manufacturing sequences, material removal is a necessary step; this is often true in the case of small workpieces, such as ultra small electrical devices commonly referred to as integrated circuits (IC's), discrete semiconductor devices (transistors and diodes) and thin-film devices such as resistors or capacitors. Where IC's or thin-film circuits are concerned, such machining often takes the form of forming small apertures or windows in both dielectric and metal films. These openings typically have dimensions in the micron range for IC's, in the 2-5 mil range for discrete devices, and greater than 5 mils or so for thin-film resistors or capacitors.
The reasons for making these openings are many, but several reasons are more typical than others.
First, in making semiconductor devices (both IC and discrete) a dielectric film is often thermally grown or deposited, over underlying circuits, which may include uni-polar and bi-polar devices, including doped regions and p-n junctions. The purpose of the dielectric film is to passivate and protect these circuits. However, electrical connection to the underlying circuits must be made through the dielectric. This is usually accomplished by making windows through the dielectric film to expose the circuits so that metal conductors can be formed by evaporation, sputtering or plating.
Second, in manufacturing many of the devices discussed above, there is often a requirement for processing steps, such as junction enhancement by ion implantation or by vapor diffusion. Again an aperture or window must be opened, in this case to expose the junction or other desired region for such a step.
Third, so-called mesa devices usually contain a moat surrounding the mesa. This moat exposes the p-n junction below the mesa's top, thus isolating adjacent mesas and their p-n junctions from each other.
Fourth, as described in commonly-assigned U.S. Pat. No. 3,400,456, reissued on Oct. 2, 1973 as U.S. Pat. No. Re. 27,766, thin-film resistors and capacitors may be adjusted to value by the formation through their metallic and dielectric films of apertures. These apertures either remove material from a film, thereby decreasing its effective area (and increasing resistance) or penetrate the film and connect it to an adjacent film, thereby increasing the effective area of the first film (decreasing resistance).
It has been found that a taper which is inward from the top to the bottom of the aforementioned apertures or windows if often desirable. Such a taper is shown in U.S. Pat. No. 3,808,069.
These tapers, where the aperture top is wider than its bottom, are desirable especially where subsequent processing steps involve steps such as irradiation or material deposition. Without the taper, "shadowing" may be caused by vertical or inwardly sloping walls. Specifically, irradiation and deposition occur as though the impinging energy or the material being deposited were emanating along a line of sight from a remote point source. If an inward, top-to-bottom taper is not present, locations at the bottom of the aperture, window, etc., may be shielded or shadowed by upper parts of the aperture wall and not receive their proper irradiation or deposited material.
It has also been found that processes such as ion implantation which require window formation to expose a surface to be treated, are best carried out when the window has an inward top-to-bottom taper. Moreover, for related reasons, moats formed in mesa devices are usually desired to have tapered walls. Lastly, it has also been found convenient to taper the holes used to adjust the value of thin-film devices.
Chemical methods are known for providing the desired tapers as described in the aforementioned U.S. Pat. No. 3,808,069 patent. These methods are slow, costly, time consuming and involve much handling which results in high-product breakage.
Laser-implemented methods of forming blind holes are also well known. See, for example, "Scribing of Al.sub.2 O.sub.3 Material by YAG and CO.sub.2 Lasers" a paper presented by U. C. Paek and V. J. Zaleckas at the fall meeting of the American Ceramic Society, Electronic Division in Denver, Sept. 18-20, 1974. Most of the methods involve a "brute-force" approach, whereby holes are literally "blasted" through a rather massive article. In some cases tapers have been observed, and these have been attributed to at least two factors:
1. Because the massive articles are quite thick the hole formed by the laser ultimately has a quite appreciable depth. That is, the hole bottom moves away or recedes from the laser during hole formation, the beam is defocused which causes less energy to be transferred to the receding hole bottom.
2. Again, because the massive articles are thick, the holes, even at intermediate formation stages, are quite deep. Molten components collect in these deep holes, leading to an inefficient transfer of energy from the beam to the hole bottom.
Both factors may tend to produce tapers because, as compared to hole formation at its earlier stages, holes formed during times of lessened or inefficient energy transfer are smaller. This tapering effect is quite unpredictable, and in fact, was felt by earlier workers to be undesirable. It was eliminated either by using very high power lasers, which form holes of appreciable depth by almost instantaneous vaporization, or by tailoring the wavelength of the laser to effect more efficient absorption of energy by the articles. These latter expedients often lead, however, to undesirable debris on the articles' surface surrounding the hole.